Definition of Terms
Used In Engine Repair, Rebuilding and Maintenance
Homepage -> Repair ->Engine
This glossary is courtesy of Chilton's Motor Age
- Align Bore
- A procedure used to re-align main bearing or camshaft bearing bores to
bring them back into alignment after warping of the block or alignment
shifts due to long-term mechanical stresses. By securing the block and then
passing a single boring tool through all the bores without changing the
alignment of the tool, the bores are restored to proper position. Properly
aligned bores ensure that each bearing will carry equal load.
- Alkaline Base (Cleaner)
- A form of chemical cleaner used in automotive parts cleaning; includes
most soaps and detergents, as well as resin-, silicate- and phosphate-based
cleaners. Most work best when heated to 180°F and above.
- (usually said of gears) Total play measured in both directions. For
example, timing gear backlash measures the total rotation of the timing gear
when forced as far as possible in each direction with the crankshaft gear
- Also sometimes described as a "journal bearing." A bearing is a
special, replaceable insert composed of several layers of such metals as
lead, tin and antimony. These metals are capable of withstanding high loads
and wearing for a long time, provided they remain coated with a thin layer
of oil under pressure. The main and connecting rod bearings of a gasoline
engine are each formed of two half-circle sections which are assembled
together to form a complete, circular bearing.
- Bearing Crush
- When journal bearings are assembled, each insert installed either in a
semi-circular bore in the block (or rod), or a similar area of a bearing
cap. When assembled into the cap and the bore section of the block, the ends
of the inserts stick out slightly. When the cap is put in position, the ends
of the bearing inserts butt up against one another before the cap actually
seats against the block. In this position, the inside diameter of the
bearing will be slightly greater when measured between the centers of the
inserts than when measured between the two ends of either insert. The
difference between these two diameters is the bearing crush. The crush is
removed when the main cap bolts or connecting rod cap bolts are torqued.
- Bearing Oil Clearance
- The difference between the outside diameter of the crankshaft bearing
journal or crankpin and the inside diameter of the fully assembled bearing.
This is the space that becomes filled with oil when the engine is started
and remains filled with oil as it runs. Bearing oil clearance is usually
measured with a crushable Plastigage insert. This clearance is critical
because too large a clearance results in low oil pressure, while too small a
clearance usually destroys the bearing because of lack of lubrication and
- Bearing Spread
- The difference between the diameter of a bearing insert when measured at
the ends prior to assembly, and after assembly into the bearing bore. The
bearing is snapped into position because of the spread.
- The act of removing large amounts of metal from a cylinder or bearing bore
in order to remove damaged metal, correct alignment problems, and increase
- (as contrasted with "bearing") Mechanically, almost identical to
a bearing. The difference is that a bushing forms a continuous circle.
Therefore, bushings are drawn in and out of continuous bores in a block,
rather than being retained by a cap.
- Creating a cupped section leading into the threads cut into a casting,
such as a cylinder block. The top of the cylinder bore is also chamfered.
The chamfer tends to prevent the metal from cracking when it is subjected to
the stress imposed when the bolt is later torqued.
- Concave Wear
- Wear pattern typical on a high-mileage valve lifter. The convex or bulged
character of a new lifter is replaced by a dished shape, in which more
material has been removed from the center area of the lifter bottom than
from the edges.
- Connecting Rod Side Clearance
- The clearance between the side surfaces of the connecting rod and cap
adjacent to the edge of the bearing and the nearby sections of the
crankshaft. This clearance must be controlled to keep the rod from shifting
from side to side in the engine while it is running, and to prevent
excessive oil flow through the connecting rod bearing. Excessive clearance
may be corrected by replacement of worn parts or, if worthwhile, by welding
material to the crankshaft and machining it down to provide the correct
- Said of a new camshaft lifter. The lifter bottom is rounded with the
lowest point occurring near the center, so it will contact the camshaft lobe
somewhere near its center, rather than wearing the edges.
- An engine's ability to trap air and reduce its volume to create pressure.
This ability is lost when engine parts such as piston rings and cylinders
wear, or when valves lose their ability to seal due to burning or mechanical
damage. Compression makes an engine fire smoothly (helps good combustion)
and operate efficiently. An engine compression test is one very effective
way of measuring wear and checking for mechanical problems.
- A situation where a smaller circle is exactly in the center of a larger
circle when measured in every direction. Valves will operate efficiently and
give long service life only if the valve guide is concentric with the valve
- Crankshaft Journal
- The sections of the crankshaft which ride on a thin layer of oil coating
the main bearing surfaces. These surfaces must be of the right diameter and
properly polished to ensure long service life.
- A cross-hatch is honed into the cylinder in the last step of finishing the
cylinder bore. The cross-hatch pattern retains oil for proper lubrication.
If the bore was perfectly smooth, lack of lubrication would damage the
cylinder bore, pistons and rings.
- A form of valve damage in which excessive heat causes the head of the
valve to become cupped because it cannot withstand the normal forces created
when it seats.
- Deck Height
- The height of the cylinder block surface on which the cylinder head rests.
Deck height is measured from the crankshaft centerline to the deck surface.
It should be uniform from front to rear and may be corrected by machining,
- Edge Loading
- Said of a new camshaft on which an old lifter has been used. The removal
of the convex characteristic from the bottom of the old lifter due to wear
will cause the outer edges of the lifter to contact, and wear, the outer
edges of the cam lobes.
- End Gap
- The distance between the open ends of a piston ring, as measured with the
ring installed in the cylinder bore. End gap is required to ensure that the
ends of the ring do not touch when the ring expands as the engine heats up.
If they do touch, the ring will typically score or break, or the engine may
seize. Excessive end gap produces excessive oil consumption and loss of
compression. End gap must be measured with the ring installed squarely in
the cylinder. Standard procedure is to position the ring at the lower end of
the area of ring travel. However, the manufacturer of the engine or parts
being used may specify another position.
- The distance the crankshaft can be forced back and forth when installed.
End-play is controlled by vertical sections of one of the main bearings and
the clearance between those sections, and an adjacent surface on the
- Free-Standing Height
- The height of a valve spring with no pressure tending to shorten it. If
valve spring height does not meet specification, it should be discarded. If
re-used, it may allow for valve bounce or improper sealing.
- A method of refinishing a crankshaft's bearing journals and crankpins.
Grinding removes significant amounts of damaged metal and produces journals
and crankpins suitable for use with undersize bearings, or bearings with a
smaller asssembled inside diameter than the standard one.
- Guttering Valve
- Guttering occurs when leakage causes small grooves to form in the valve
face due to the passage of hot gases. A common cause of guttering is poor
seating. The valve face near the channel eventually burns away.
- A method of removing metal from a cylinder bore. Honing is less aggressive
(removes metal more gradually) than boring. The final step in finishing a
cylinder is finish honing, which removes very little metal, and is used
primarily to create the proper surface.
- Hoop Stressing
- A form of valve failure that occurs due to overheating. Cracks develop on
the rim of the valve face because the center cools much more slowly than the
outer edge. Hoop stressing may be a symptom of combustion or cooling
problems, or it may result because the vehicle is working against a load
greater than that for which it was designed.
- Impact Failure
- A form of valve damage that occurs when the valve seats too hard or hits
the piston. It can occur due to a broken timing chain, faulty keeper
installation, or operating the engine at excessive rpm.
- It may be necessary to install a valve seat insert or insert guide if a
valve seat or guide surface in a cylinder head is worn excessively. In other
words, there is not enough undamaged metal remaining to create a guide or
seat of suitable dimensions. The head is bored out to the proper diameter,
then the new part is pressed in and must be finished. Some aluminum cylinder
heads use insert guides and/or valve seat inserts because the head material
is not hard enough to ensure good wear characteristics.
- Installed Height
- (said of a valve spring) The vertical distance from the valve spring pad (where
the bottom of the valve spring rests on the cylinder head), to the lower
edge of the valve spring retainer. Incorrect installed height can cause
improper sealing or valve bounce if the dimension is too great. If installed
height is not great enough, there may be damage to the valvetrain due to
having the spring coils contact each other and put excessive strain on the
- A process which raises or displaces metal inside a worn valve guide to
restore it. The inside diameter is reduced and then the guide is bored out
to the proper dimension. The end result is spiral grooves that provide
excellent lubrication and allow operation at a very small stem-to-guide
- Magnetic Particle Detection
- A process used to find cracks in cast iron heads, especially the
combustion chambers. A powerful magnet is positioned directly above the area
to be inspected. Then, an iron powder is sprinkled over the area. If there
is a crack, it will interrupt the magnetic field and produce a broken field
pattern, rather than the single pattern that is normally visible. The check
should be made twice, at 90 angles in case a crack would align with the
field lines in such a way as to prevent detection.
- A process of quickly removing with carbide cutters a thin layer of metal
from the surface of a head or block deck. This is done to restore the
flatness of the surface.
- A thinning of the valve stem where it passes out of the head at the bottom
of the guide. Abrasives or high temperatures may wear the stem surface,
thinning it and possibly causing the valve head to break right off.
- Open/Closed Pressure
- Two of the tests done to determine the ability of a used valve spring to
perform satisfactorily. Spring force is precisely measured at two different
dimensions, one representing the normal valve-closed position, and the other
representing the wide-open position. If the valve spring is square, stands
at the right height and passes these two tests, it may be re-used.
out-of-round - the difference between the cylinder bore measurement parallel
to the crankshaft and perpendicular to the crankshaft.
- Positive Crankcase Ventilation (PCV)
- A system that draws fresh, filtered air through the crankcase to remove
blowby gases. The gases are then pulled into the intake manifold to be
reburned. The PCV valve controls the airflow so it will be stable,
regardless of intake manifold vacuum. A malfunctioning PCV system can cause
engine oil leaks and high crankcase pressure, mimicking more severe engine
- A process used to remove grinding "fuzz" or roughness remaining
on the journals and crankpins after a crankshaft has been ground to size.
The shaft will typically be micro-polished with a 320 grit belt to 0.0001
in. by moving the crankshaft in the normal direction of rotation.
- Pyrolitic Oven Cleaning
- A cleaning process that uses heat. Parts are baked at 400-800F, causing
deposits to harden and flake off or to burn off.
- A process used to remove metal from the inside of a valve guide and
increase the guide hole size. It may be done to allow the use of oversize
valve stems, restore the diameter after knurling, or finish a new insert
guide to standard inside diameter.
- Ridge Reaming
- A process used to remove the ridge worn into the top of the cylinder at
the top of ring travel. The ridge need not be removed if the cylinder is to
be rebored and oversize pistons installed.
- A measurement taken to check concentricity of a valve seat or squareness
of a camshaft drive gear. To check a valve seat, a special instrument which
rides in the valve guide is installed and zeroed with its stem resting
against the valve seat. It is rotated 360 and the highest measurement
recorded to determine seat runout. To check a camshaft drive gear, a dial
indicator is mounted along the axis of the camshaft with its stem resting
against the outer edge of the gear. The indicator is then zeroed, the
camshaft rotated 360 and the highest measurement recorded to determine
- A thin metal ring used to adjust (shorten) valve spring installed height.
The shim is installed between the bottom of the valve spring and the spring
seat on the cylinder head. Multiple shims of different thickness may be used
to get just the right height. In this case, the thickest shim should be
installed next to the spring.
- Side Clearance
- The clearance between the side of a piston ring and the side of the ring
groove. Side clearance is checked with a feeler gauge and the ring to be
used in the assembled engine. If clearance meets specifications, the piston
may be re-used. If clearance is excessive (the groove is too wide due to
wear), the piston will have to be replaced.
- Spring Squareness
- A check made to ensure that a used valve spring will produce even pressure
all around the valve spring retainer. A spring that is not square will cause
wear of the valve stem and guide. Spring squareness is checked with a
- Stem-To-Guide Clearance
- A precise measurement of the clearance between the valve stem and the
guide. To take the measurement, the valve is inserted into the guide and a
dial indicator is used to measure total motion of the valve tip as the valve
is rocked back and forth in the guide. The clearance also may be determined
by measuring stem outside diameter and guide inside diameter, and
subtracting the smaller measurement from the larger one. This clearance must
be great enough to ensure adequate lubrication, but small enough to prevent
excessive oil consumption.
- The difference between the diameter of the cylinder bore at the bottom and
at the top, just below the ridge.
- A type of head bolt. Torquing it to the correct figure actually causes the
bolt to permanently stretch a small amount, but maximizes sealing force.
This type of bolt must never be re-used.
- Said of a valve that does not remain tightly seated, but actually bounces
after it closes. This commonly occurs because the valve spring is too weak.
It may be accompanied by hydraulic pumping-up of the lifter, which creates
poorer and poorer seating on successive closing cycles. It also may occur
when a camshaft with a high lift and a rapid opening-and-closing profile has
been substituted for a standard cam without modifications to the valve
- Valve Contact Line
- The ring on the valve head and seat that represents the actual location
where metal parts touch one another when the valve is lightly seated. Proper
machining of seat and valve face width and angles will position the contact
line 1/32 in. above the valve margin.
- Valve Lapping
- A process where the valve face is coated with a grinding compound and the
valve rotated against the seat. This causes the surfaces to more perfectly
mate with one another. Lapping is done after the valve face and seat are
machined, or if the engine shows very little valve and seat wear.
- Valve Lash
- The clearance between the tip of the valve stem and rocker. Valve lash is
always measured with the lifter positioned off the lobe of the cam. This
measurement is taken while adjusting a solid lifter valvetrain, or to make
sure hydraulic lash adjusters can adjust properly. Hydraulic lash adjusters
must have their pistons bottomed in the lifter bores in order for this
adjustment to be checked. Improper valve lash can cause valvetrain noise or
- Valve Margin
- The distance between the valve's lower surface and the bottom of the face
(the straight vertical section at the bottom of the valve head). When
re-machining the valve face, it tends to get wider, and the machined area
may get too close to the bottom of the valve, weakening its structure and
leaving a sharp edge that is vulnerable to heat. There must be 3/64 in. of
margin on older valves, and 1/32 in. on valves made of the latest materials.
- Valve Seat Recession
- A condition that occurs when the seating area of the head naturally
recedes into the head due to pounding that occurs as the valve seats. Rapid
valve seat recession becomes a problem when unleaded fuel is used in an
engine without induction hardened seats. It also may be caused by valve
spring tension that is too great. Seat recession is typically repaired with
an insert seat.
This page last updated
April 28, 2001. Send us your feedback,
and come join the Imperial
Mailing List - Online Car Club